It’s entirely possible to get these fasteners mixed up. I’m not a manufacturing expert, but I did walk the assembly line when it was early in production, and it just amazes me that we can build something this complex. In retrospect there should have been some fail-safe way to avoid this, or some very serious training.
Aircraft don’t usually have sufficient weight margins to allow a 2x safety margin, which is more appropriate for a bridge. I’m skeptical of that claim.
Galvanic corrosion was a big problem with the F-22. I don’t think that this would be a problem in this case, but I could be wrong.
There needs to be a study of where these fasteners might be. If one lot got mixed up, that could be straightforward to fix. If there is some random and unknown mixing for an unknown amount of time, then that won’t be good. The most likely corrective action will be a stringent inspection schedule of operational planes. That cost is indirect and buried in the normal operating cost of the maintenance by the service.